This invention is directed to a method for simultaneously removing mercury and water from fluids by contacting the fluids with porous adsorbent substrates having elemental silver and/or gold impregnated or coated thereon. This invention further relates to a multi-layered guardbed having at least two distinct adsorbing regions each of which has a porous substrate, such as zeolite A, as the adsorbent material whereby the porous substrate of the first distinct region is mixed, coated or impregnated with elemental silver and the porous substrate of the second distinct region is mixed, coated or impregnated with elemental gold.
It is often desirable to remove water from fluids, such as from hydrocarbon fluids, an example being natural gas. Water can be effectively removed from such fluids by means of molecular sieves, particularly the synthetic crystalline zeolite known as zeolite A. Zeolite A contains cavities formed by sodalite cages stacked in simple cubic form. These cages are made up of truncated octahedra having a silica or alumina tetrahedron at each point. The cavities are surrounded by eight oxygen atoms, and are partially blocked by cations that balance the charge on the oxygen atoms. In zeolite A, each alumina moiety is balanced by two positive charges. If the cation is sodium, the cavity is reduced to about 4.2 angstroms in diameter. If the cation is potassium, the cavity is reduced to about 3 angstroms in diameter. If the cation is calcium, the cavity is reduced to about 5 angstroms in diameter. Thus, zeolite A having sodium, potassium and calcium ions are known respectively as zeolite 4A, zeolite 3A and zeolite 5A. The pore diameters of zeolite A make them especially suitable as drying agents, since the pores are large enough to accommodate water molecules, but not most other molecules found in nature.
When a zeolite used for drying fluids becomes saturated with water, it must be regenerated, which is often accomplished by heating with flowing hot gas. Zeolite 4A is the most commonly used molecular sieve for this purpose.
It is also important to remove mercury from fluids such a natural gas, liquid condensate, oil and waste waters. For example, natural gas may contain as much as 250 ppb (micrograms/m.sup.3) mercury. Following the drying procedure described above, the natural gas is, in many commercial liquefaction operations, transported to aluminum heat exchangers. Mercury present in the natural gas causes corrosion of the aluminum and must therefore be removed.
Various methods for removing mercury from fluids such as natural gas are available. For example, U.S. Pat. Nos. 4,101,631 and 4,474,896 describe the removal of mercury from gas streams by means of sulphur or sulphur compounds on supports such as zeolites and activated carbon. Such methods are capable of reducing the level of mercury to about 0.1 ppb. Even this level of mercury in a stream, however, can injure aluminum heat exchangers.
U.S. Pat. No. 4,892,567 discloses a method for simultaneously and repeatedly removing mercury and water from a hydrocarbon fluid by contacting the fluid with an effective amount of a molecular sieve comprising zeolite A and 0.001-15% elemental silver or gold. Additionally, this reference discloses a regenerable molecular sieve comprising 0.001-15% elemental silver or gold in or on zeolite A.
It has recently been discovered, however, that a significant fraction of mercury in the natural environment is organic mercury rather than metallic mercury. Metallic and inorganic mercury are easy to remove using conventional methods, including silver. Organic mercury, on the other hand, is very difficult to remove using conventional methods. While both forms of mercury exist, organic forms such as dimethyl mercury, diethyl mercury and the like are more difficult to remove using conventional mercury removal techniques and are significantly present in the residual mercury of some natural gases. These difficult to remove organic mercury compounds are called refractory mercury. In conventional mercury removal processes, such as those using sulphur compounds on activated carbon supports, such residual mercury is generally left even after the removal of easy to remove mercury is complete. Additionally, the nature of the mercury compounds may be different for a given natural gas source, further complicating the removal process. The reactivities of various mercury compounds with metals such as gold, silver, copper and iron varies greatly. It has recently been discovered that organic mercury compounds such as dimethyl mercury react with gold quite readily. In contrast, however, organic forms of mercury react little if at all with silver. Thus, it is difficult using prior art methods to remove mercury to a low level from a fluid such as a gas using silver alone due to the presence of such refractory organic mercury compounds. For example, a molecular sieve containing silver is inadequate to reduce the level of mercury to a low level useful for commercialization if the natural gas contains both metallic and organic mercury.
There is a need to reduce the level of mercury in fluids to below 0.01 ppb or less. In order to be commercially feasible, the method must be inexpensive as well as efficient. Since water most often must also be removed from hydrocarbon fluids, it would be especially desirable to be able to remove mercury and water simultaneously and repeatedly from a fluid with the same agent, in one reactor or drier, such that once the fluid has contacted the agent, the level of mercury is less than 0.01 ppb and the level of water is less than 1 ppm. A composition of matter capable of simultaneously removing mercury and water from a fluid must be able to be regenerated many times simultaneously for both of these purposes in order to be considered efficient enough to be commercially feasible. While the use of gold alone would solve the problem of removing both inorganic and organic forms of mercury, the cost associated with the quantities of gold necessary to commercialize the process would be prohibitive. Thus, the use of other metals capable of removing the inorganic mercury species in combination with gold serves to significantly lower the cost concerns, thereby making commercialization feasible, as well as solving the technical problems associated with removing water and both forms of mercury simultaneously . The present invention thus makes selective use of the gold- and silver-containing adsorbent regions of the inventive drier-bed.
Therefore, there is a need for a process and adsorption device which is useful for simultaneously and repeatedly removing metallic and organic forms of mercury as well as water form various fluids. Thus, one principal object of the present invention is to provide such a process and device using porous materials such molecular sieves having silver and gold mixed with, coated on or impregnated therein and having distinct layers or regions of gold and silver in the order of silver and gold to remove the metallic and organic mercury compounds respectively.